AllMusic Genres is a plugin for MusicBrainz Picard that scrapes genre, style
and potentially other information from allmusic.com

In case you downloaded it from somewhere else, you can always get the latest
version of this plugin from http://github.com/daveisadork/picard-allmusic

At this point, this plugin requires the following library:

 * BeautifulSoup 
   http://www.crummy.com/software/BeautifulSoup

The first step is to create a folder called allmusic in your Picard plugins
directory (mine is ~/.config/MusicBrainz/Picard/plugins) and put __init__.py in
there. If you're on Linux, you'll need to install the python-beautifulsoup 
package provided by your distribution (to do this on Ubuntu, you could just run
'sudo aptitude install python-beautifulsoup' at a terminal) and you're good to
go. If you're on Windows, I would think about switching to Linux if I were you. 

There are no configuration options yet (but I'm Working on that), so you're kind
of just stuck with whatever the plugin happens to find. Basically, assuming we 
can find a page for whatever album we're trying to tag, we just grab all of the
"styles" listed on that page and tell Picard to use them as genre tags. Failing
that, we grab the "styles" from the artist page and use those. 

On my music collection, I ended up with about 96% coverage, meaning only about 
4% of my music overall was either not tagged with any genre data (common with 
non-score movie soundtracks and other compilations) or was tagged with incorrect
genre data (*very* uncommon, though I am working on a way to deal with it). 

Also, it's really slow. So I wouldn't recommend trying to run it on your 40k
song collection all at once. Although, after dropping urllib and urllib2 in
favor of Picard's own facilities, things aren't so bad.